Harder began fleet operations out of New London in June
1953. Shortly thereafter Harder departed for the British Isles.
During this voyage, she experienced mechanical difficulties with
her then-experimental Fairbanks type 3 engines. In August 1953, her
engines broke down completely off the east coast of Ireland. She was taken under
tow by USS Tringa and
endured the longest tow in submarine history, 2100 miles (3400 km)
across the Atlantic to New London, Connecticut.

During the next several years Harder carried out a
schedule of training and readiness operations with ships of the
Atlantic Fleet and Allied NATO
nations. Operating from the North Atlantic to the Caribbean Sea, she
engaged in sonar evaluation
tests, supported ASW tactical exercises, and participated in
submerged simulated attack operations. In March 1959 she
participated in SUBICEX, during which she cruised 280 miles (450
km) beneath the ice packs off Newfoundland, further than any
conventionally powered submarine had previously gone.

After completing a three-month advanced submarine exercise in
the Atlantic, Harder changed her home port to Charleston, South Carolina,
where she arrived 17 November and joined Submarine Squadron 4. She
continued operations in the Atlantic and Caribbean for more than a
year, then departed Charleston 24 May 1961 for deployment off the
western coast of Europe and in
the Mediterranean Sea. After reaching Bremerhaven, Germany, on 9 June, she
supported ASW operations with ships of the Federal German Navy.
The following month she steamed via Holy Loch, Scotland, and Rota, Spain, to the Mediterranean
Sea where she supported peace-keeping operations of the 6th Fleet. Harder
returned to Charleston 28 August.

Training exercises and tests evaluating ASROC and other ASW systems
kept Harder busy until she entered Charleston Naval Shipyard for overhaul in
October 1962.

Ready for action in April 1963, she operated along the Atlantic
Coast and in the Caribbean Sea for the next 2½ years
polishing her underwater warfare tactics and operating with destroyer-type ships as
they worked on ASW exercises.

Harder again entered Charleston Naval Shipyard on 22 October for
a thorough overhaul and modernization in which she received an
18-foot hull extension to accommodate the new PUFFS passive sonar system, a redesigned
superstructure, new engines as well as improved electrical and
electronic equipment. Modernization completed early in 1967,
Harder rejoined the fleet.